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diff --git a/src/tools/clippy/src/docs/manual_retain.txt b/src/tools/clippy/src/docs/manual_retain.txt
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index 000000000..cd4f65a93
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+++ b/src/tools/clippy/src/docs/manual_retain.txt
@@ -0,0 +1,15 @@
+### What it does
+Checks for code to be replaced by `.retain()`.
+### Why is this bad?
+`.retain()` is simpler and avoids needless allocation.
+### Example
+```
+let mut vec = vec![0, 1, 2];
+vec = vec.iter().filter(|&x| x % 2 == 0).copied().collect();
+vec = vec.into_iter().filter(|x| x % 2 == 0).collect();
+```
+Use instead:
+```
+let mut vec = vec![0, 1, 2];
+vec.retain(|x| x % 2 == 0);
+```
+\ No newline at end of file